Carburization of metal articles



1957 F. P.. ANDERSON ETAL 2,804,411

CAR'BURIZATION OF METAL ARTICLES Filed Dec. 22, 1955 L z E #123115 $70547 2?. Anaewofl a Z A. Kiflg A r I I v v v v A r a a u a u I v v I I I f v ff! r A a a a r a I a v v r I r r a I v a;

DJJA 1 f United 7, 2,804,411 Patented Aug. 27, 1957 nice cARnuRrZArroN or WTAL ARTICLES Floyd R. Anderson, Denver, C010" and Jack A. King,

St. Catharines, Untario, Canada, assignors to Gardner- Denver Company, Denver, Colo, a corporation of Delaware This invention relates to a process for case hardening hollow metal members, and more particularly, to an improved process for case hardening both the interior and exterior of such members which may have difficulty accessible interior surfaces.

Although the instant invention may have application in a number of fields, particularly those involving the case hardening of both the interior and exterior surfaces of elongated hollow articles having relatively thin body portions separating the interior and exterior surfaces, the instant invention is particularly adapted to the formation of a novel drill rod structure. The so-called rock drill rods have generally accepted standard sizes ranging from about 2 to about 20 feet in length and from about 0.75 to 2 inches in diameter. Actually, hexagonally cross-sectioned rods are usually employed with maximum cross-section dimensions of inch to 1% inch (but such dimensions are considered to be diameters of a substantially round cross-section for the purposes of the instant description). The rods are formed from steel stock by hot or cold Working usually over a suitable mandrel, which results in the formation of an approximately 4 inch axial bore in the rods. Although the particular manner in which the bore is formed in the rods is not so important, this bore does extend the entire length of the rod and the ratio of bore length to bore diameter is very substantial.

Heretofore, drill rods have been made of high carbon steels which are difiicult to Work; but it has only recently been discovered that drill rods giving distinctly superior performances may be made from steels having a lower carbon content which are case hardened on both the outside surface and the inside surface defining the axial bore. A number of difliculties, however, have been encountered in attempting to fabricate such a case hardened drill rod. In general, the case hardening process involves first a cementing or carburizing step and then a quenching step wherein the hot carburized article is immersed in a quenching liquid such as water or oil so as to quickly cool the article from substantially the carburizing temperature to substantially room temperature (or at least to a temperature of about 300 F. so that further metallurgical changes do not occur on cooling to room temperature). The carbon content of the :carburized surface portions as well as the rate at which such surface portions are quenched will determine the hardness and other desirable properties in the case hardened layer. Also, difierent quenching or cooling rates for different surface portions may make such a significant difference in the resulting material that flaws or defects may be created, if such surface portions are adjacent. Also, excessive warpage, over and above that which may be obtained using ideal carburizing and quenching techniques, may result from lack of uniform quenching. A peculiarity of drill rods and similar hollow articles having difiiculty accessible interior surfaces is that uniform quenching of such surfaces by contact with the quenching liquid cannot be obtained. For example, the initial quenching liquid entering the bore is only a very small amount of liquid which engages a rather substantial amount of heated surface and this results immediately in vaporization of the quenching liquid which, in the case of such drill rods, has been found to violently force vapor and liquid out of the opposite end of the bore. Such a fast flowing stream of heated steam or oil is very hazardous and difficult to handle. In addition, the surfaces of the bore are not uniformly contacted with liquid, because of the presence of vapor in the bore, so that certain of the difficulties hereinbefore mentioned are en countered.

The instant invention affords a unique solution to the problems peculiar to this art. Although quenching by actual surface contact betweenthe quenching liquid and the surface to be case hardened has heretofore been considered a necessary aspect of the procedure, the instant invention provides a process which avoids this procedural step with respect to the difficulty accessible interior surfaces of the article, while still obtaining case hardening thereof. In other words, the instant invention contemplates quenching the rod by contacting the outside surface only thereof with the quenching liquid, while preventing contact between the quenching liquid and the inside surface. This is accomplished by sealing the open ends of the bore before quenching; and the instant invention is based in part on the discovery that the somewhat less rapid cooling of the interior surfaces by heat conduction through the thin shell or body of the rod results in not only an extremely uniform but also a reasonably effective case hardening of the interior surface. The instant invention is further based on the discovery that additional case hardening (particularly on the inside surface) may be obtained by a specific subsequent chilling process involving temperatures farbelow room temperature, so as to obtain superior case hardening.

The uniformity of the interior surface case hardening has been found to be particularly important from a strength and performance point of view. In addition, this particular type of quenching operation (prior to the aforementioned chilling operation) results in the formation of an interior (and exterior) case hardened surface which does not fracture or develop defects during the relatively slight amount of cold working that is required to straighten out the rods and eliminate the warpage inherent in case hardening operations of this type. Because of this the rods may be readily straightened by cold working and then chilled to impart the superior case hardening (which resists such a straightening operation).

It is, therefore, an important object of the instant invention to provide an improved case hardening operation for hollow drill rods and the like articles.

It is a further object of the instant invention to provide an improved method of producing a steel drill rod having an axial bore that comprises carburizing the inside surface of the rod defining the bore and also the outside surface of the rod, sealing the ends of the bore, quenching the rod by contacting the outside only of the rod with quenching liquid, whereby cooling of the inside surface is accomplished by heat conduction through the body of the rod, and then chilling the rod to temperatures within the range of 0 to minus F.

Other and further objects, features and advantages of the present invention will become apparent to those skilled in the art from the following detailed disclosure thereof and the drawings attached hereto and made a part hereof.

On the drawings:

Figure l is a sectional elevational view of a case hardened drill rod made by the process embodying the instant invention, and shown with sealing means closing the open ends of the bore;

Figure 2 is an enlarged cross-sectional view of the drill rod of Figure 1 taken substantially along the line IIII of Figure 1; and

' Figure 3 is a cross-sectional view comparable to Figure 2 but taken on a drill rod having a different crosssectional shape.

As shown on the drawings:

In Figure 1, the reference numeral indicates generally a drill rod embodying the instantinvention' at the completion of the quenching step. The drill rod 10 has a case hardened layer 11 defining its outer peripheral surface, case hardened layers 12 and 13 defining, respectively, the top and bottom end surfaces of the rod 10 and a case hardened surface portion or layer 14 defining an axial bore B extending the full length of the rod 10. Actually, the rod'10 comprises a first case hardened sleeve 14 defining the axial bore B, a second case hardened sleeve 11 defining the outer surface for the rod 10 and a steel body 15 interposed between the first and second case hardened sleeves 11 and 14. This structure has been found to be uniquely satisfactory for dr-ill rods.

As shown in Figure 1, the overall length l of standard drill rods may range from 2 to feet, whereas the maximum cross-sectional dimension or diameter D may be from A: to 2 inches and the diameter d of the bore B is customarily about A inch. Although it is standard commercial practice to employ a bore diameter d of approximately fii inch, it will be appreciated that this dimension d may range from as little as about /s inch to as much as about /2 inch for the purposes of the instant invention. Because of the manner in which the bore B is usually formed during working of the steel, it is ordinarily not perfectly circular but only approximately so and the dimension d represents generally the average diameter (or corresponding average cross-sectional dimension).

As will be noted from Figure 2, the overall crosssection of the rod 10 is hexagonal and the maximum cross-sectional dimension D is the dimension hereinbefore indicated and, for the purposes of the instant invention, the dimension D may be considered to be a diameter of a substantially round cross-section (or substantially the average cross-sectional dimension). As will be noted in Figure 3, the rod 10 has a circular cross-section whose diameter D is the same as the diameter D hereinbefore described in connection with Figures .1 and 2, but the remaining elements of the rod 10 of Figure 3 are all functionally the same as the corresponding parts or elements in Figures 1 and 2 and are thus designated with primed reference numerals to correspond. As will be seen, the rod of Figure 3 is made by the same process as that which will be described for the fabrication of the rod of Figures 1 and 2.

The ratio of the bore length l to the bore diameter (on the basis of the standard bore diameter of inch) ranges from a minimum of about 100:1 to a maximum of about l000:1 (although the maximum is actually unlimited for practical purposes). The minimum ratio of 100:1 for Ice! has been found to be generally definitive of a difficultly accessible interior surface for the purposes of the instant invention, in that problems in quenching are definitely apparent at this minimum ratio and for any larger ratio. Another feature here involved is that the wall thickness t (Figure l) for the rod portion surrounding the bore B may range from about inch to a maximum of about 1 inch in order to obtain the best chilling or quenching effect on the case hardened layer 14 by the instant quenching process. Wall thicknesses substantially greater than this dimension will serve to slow down the cooling through the body of the rod to too substantial an extent to obtain the best results.

The first step in the practice of the instant invention is the carburizing step wherein both the inside surface layer 14 0f the rod and the outside surface layer -11 of the rod are carburized. Carburization (sometimes referred to as cementation) is a well known process, which comprises exposing the surface to be carburized to a carburizing material at carburizing temperatures. The carburizing material may be solid or fluid. For example, charcoal and/ or coke in combination with chemical energizers in particulate compositions such as are described in Rodman U. S. Patents No. 949,448 and 1,432,416 may be employed using the technique described by Rodman. Also, a bath of a salt containing carbon, such as a potassium cyanide salt bath may be employed as a liquid carburizing material; or a hydrocarbon such as natural gas may be employed as a gas carburizing material (although the gas is less practical for carburizing the difficultly accessible interior surfaces here involved because of the difficulty in obtaining uniform carburizing gas compositions throughout the bore B). In such carburizing processes the carbon begins to penetrate the steel surfaces at about 1300 F., but temperatures up to as much as 2000 F. may be used to accelerate the process. Preferably, a carburizing temperature of 1650 to 1700 F. is employed.

The time during which the ferrous metal surface is exposed to the carburizing material at the carburizing temperature will, of course, determine the depth of the carburized layer and the depth of the resultant case hardened layer. For the purposes of the instant invention the carburized or case depth 0 and C on the inside and outside surfaces, respectively (Figure 2) may range from about 0.010 to about 0.125 inch, although the preferred case depth 0 and/ or C for use in the practice of the instant invention in the fabrication of drill rods is 0.050 to 0.090 inch. Expressed on the basis of the rod diameter D, the case thicknesses c and C are each preferably 005-007 D. The carbon content in these carburized portions is 08-12%.

As a typical example, using a 1 /2 inch hexagonal crosssectioned SAE 4320 steel, 20 foot drill rod having a A inch axial bore, the rod is completely embedded in (so as to contact all surfaces with) a solid carburizing material, in particulate form suitable to pass an 8 mesh screen, composed of coked charcoal or coal parts by weight), barium carbonate (5 parts by weight) and sodium carbonate (5 parts by weight), and maintained at 1700 F. for ten hours. This results in carburized layers 11 and 14 on the outside and inside surfaces having a car burized depths C and c each of 0.080-0.090.

Comparable results are obtained by maintaining the same rod immersed in a molten potassium cyanide bath for the same period of time and at the same temperature.

If either of the foregoing procedures is repeated except that a A; inch hexagonal cross-sectioned SAE 8620 steel 10 foot drill rod having a inch axial bore is employed at a carburizing time of 6 hours, the resulting carburized outside and inside depth C and c are each (1.05-0.06 inch.

Diffusion periods of l to 2 hours may be used to advantage (during which the carburized rods are subjected to carburizing temperatures in an inert atmosphere to make the carbon concentration more uniform in the carburized layer).

The sealing of the opposite ends at the axial bore 8, in each case, may be accomplished (at any time before quenching) merely by force-fitting plugs 16 and 17, respectively (Figure 1), therein. Such plugs may be made of steel or other suitably refractory material, but are preferably made of steel so as to stand up under the shock of the quenching operation. Quenching in the customary manner from the carburizing temperature (or from a temperature of at least about 1500 F.) is carried out by plunging the carburized rods into quenching oil (or optionally water). During the quenching, the rods are quickly I cooled to a temperature which may be considered to room temperature, in that the temperature is low enough (perhaps 300 F. or less) that further significant metallurgical changes do not take place on cooling to room temperature.

After quenching of the carburized rods just described,

the plugs are removed; and it is found that the tensile strength of the body 15 is about 125,000 p. s. i. whereas that of the case hardened layers 11 and 14 is about 300,000 to 400,000 p. s. i. (and the Rockwell C hardness is 55-63, as compared to 35-43 in the starting material).

The resulting rods are then straightened by (comparatively easy) cold working to eliminate the slight warpage, if any; and the rods may be given customary subsequent heat treatments to remove strains, etc.

The rods thus obtained are suitable for use in many drilling applications. The instant invention, however, comprises still another process step which imparts superior case hardening to the rod; and this is the final chilling process. Prior to the chilling process cold working of the rods to straighten the same can be accomplished with relative ease and without damage to the interior or exterior layers 14 and 11. Because of the quenching operation, the exterior layer 11 is more greatly hardened; but this layer 11 is supported better by the rod body 15 that is the inside layer 14 where there is greater chance of damage. Also, the relatively weaker inside layer 14 is more capable of responding (without damage) to the forces incidental to the straightening operation. The quenched rod may thus be pictured as a soft steel body 15 between an extremely hard outer sleeve 11 and a less hard inner sleeve 14 (which accommodates stretching and/or compression forces more readily). After straightening and chilling, however, the inner sleeve 14 is equally hard and the resulting rod very drastically resists the forces incidental to straightening.

In the chilling operation, the rods hereinbefiore described are immersed in a fluid maintained at at least as low as 0 F. Actually fluid temperatures of at least about minus 50 to minus 70 F. should be used for best results, but additional hardening can be noticed using temperatures as high as 0 F. About minus 100 F. is a practical minimum temperature, since lower temperatures are difficult to obtain commercially and are not additionally helpful. As an example, the rods herein described are immersed in Dry Ice in gasoline (minus 50 to minus 70 F.) in order to cool the same to the liquid temperature and to increase the interior case hardening and produce rods very resistant to cold working. The rods give excellent drilling performance (having Rockwell C hardness of 60-65).

It will be appreciated that the martensite formation during the instant quenching operation using any ferrous metal article will be retarded and austenite will be retained to a certain greater extent on the inside than on the outside of the rod because of slower cooling of the inside. The instant invention, however, contemplates the preferred use of so-called martensite retarders, so as to retain 20-30% austenite in the inside layer 14.

Although the instant invention may be employed using any ferrous materials, certain materials are preferred for drill rods. As mentioned, heretofore, drill rods have been made of high carbon steels such as SAE 1080 (specification: 0.75-0.88% C, 0.60-0.90% Mn, 0.040% P maxi-- mum, 0.050% S maximum, remainder Fe). The steels preferred for use in the practice of the invention have relatively low carbon contents of 0.10-0.30% C and may contain 0.40-1.00% Mn (plus 0.040% P max., 0.040% S max., and 0.20-0.35% Si); but these steels also contain small amounts of one or more elements such as Ni, Cr and Mo which retard the martensite formation (or which serve to retain at least 20-30% austenite in the carburized rod inside surface upon quenching). The amounts of such elements are 0.40-3.75% Ni, 0.40-1.10% Cr and/or 0.15- 0.30% M0. The preferred steel for use in the invention is SAE 4320 (or AISI 4320), specification: 0.17-0.22% C, OAS-0.65% Mn, 0.040% P max., 0.040% S max, 0.20- 0.35% Si, 1.65-2.00% Ni, 0.40-060% Cr max., 0.20- 0.30% Mo, in the case of most sizes of drill rod stock; but for the smaller size stock of one inch diameter or less SAE 8620 (or A181 8620) is preferred and this has a 6 lower Ni content and slightly higher Mn content, specification; 0.18%-0.23% C, 0.70-0.90% Mn, 0.040% P max., 0.040% S max., 0.20-0.35% Si, 0.40-0.70% Ni, 0.40-0.60% Cr and 0.15-0.25% M0. The additional Mn in SAE 8620 compensates for the reduced Ni in retarding the martensite formation.

It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention.

We claim as our invention:

1. A method of producing a steel drill rod having an axial bore that comprises heating the rod to carburizing temperature while exposing both the outside surface thereof and the inside surface defining the bore to carburizing material to carburize the inside and outside surfaces, quenching the rod by contacting the outside surface with quenching liquid while preventing contact between the quenching liquid and the inside surface, and then chilling the rod to temperatures within the range of 0 to minus F.

2. A method of producing a steel drill rod having an axial bore that comprises carburizing the inside surface of the rod defining the bore and also the outside surface of the rod, sealing the ends of the bore, quenching the rod by contacting the outside only of the rod with quenching liquid, cold working the rod to straighten the same, and then chilling the rod to temperatures within the range of 0 to minus 100 F.

3. A method of producing a steel drill rod having an axial bore that comprises carburizing the inside surface of the rod defining the bore and also the outside surface of the rod, sealing the ends of the bore, quenching the rod by contacting the outside only of the rod with quenching liquid, cold working the rod to straighten the same, and then chilling the rod to temperatures within the range of 0 to minus 100 F., said steel rod having 0.10-0.30% C, 0.60-0.90% Mn content plus a small amount of a martensite retardcr sufiicient to retain at least 20-30% austenite in the carburized rod inside surface after quenching.

4. A method of producing a steel drill rod having an axial bore that comprises carburizing the inside surface of the rod defining the bore and also the outside surface of the rod, sealing the ends of the bore, quenching the rod by contacting the outside only of the rod with quenching liquid, cold working the rod to straighten the same, and then chilling the rod to temperatures within the range of minus 50 to minus 70 B, said steel rod having (HO-0.30% C, O.40-0.90% Mn, GAO-3.75% Ni, 0.40-1.10% Cr and 0.l5-0.30% Mo content.

5. A method of producing a steel drill rod having an axial bore that comprises carburizing the inside surface of the rod defining the bore and also the outside surface of the rod, sealing the ends of the bore, quenching the rod by contacting the outside only of the rod with quenching liquid, and then chilling the rod to temperatures within the range of minus 50 to minus 70 F., said steel rod having 0.10-0.30% C, 0.40-0.90% Mn, 0.403.75% Ni, OAS-1.10% Cr and 0.l5-O.30% Mo content.

References Cited in the file of this patent UNITED STATES PATENTS McQuaid June 19, 1923 Snyder Apr. 23, 1946 

1. A METHOD OF PRODUCING A STEEL DRILL ROD HAVING AN AXIAL BORE THAT COMPRISES HEATING THE ROD TO CARBURIZING TEMPERATURE WHILE EXPOSING BOTH THE OUTSIDE SURFACE THEREOF AND INSIDE SURFACE DEFINING THE BORE TO CARBURIZING MATERIAL TO CARBURIZE THE INSIDE AND OUTSIDE SURFACES, QUENCHING THE ROD BY CONTACTING THE OUTSIDE SURFACE WITH QUENCHING LIQUID WHILE PREVENTING CONTACT BETWEEN THE QUENCHING LIQUID AND THE INSIDE SURFACE, AND THEN CHILLING THE ROD TO TEMPERATURES WITHIN THE RANGE OF 0 TO MINUS 100*F. 